In offset quadrature phase shift key suppressed carrier modulation systems (OQPSK) there are four nominal phase possibilities, depending upon the nature of a transmitted binary coded message. If there is no original transmitted reference or a signal dropout at a receiver, a four-fold ambiguity can exist. One conventional method of handling the absence of an original transmitted reference or a signal dropout involves differential coding in each of two quadrature channels that carry in phase and quadrature (I and Q) symbols representing a pair of sequential binary data bits. The differential coding introduces degradation since a single channel error results in a pair of sequential errors in the respective quadrature channels.
It is, accordingly, an object of the present invention to provide a new and improved method of and apparatus for transmitting and coherently receiving an offset quadrature phase shift key signal wherein an error in one channel does not have an adverse effect on two recovered bits of a combined serial data stream.
Another object of the invention is to provide a new and improved system for and method of transmitting and receiving a single channel of binary data modulated on an offset quadrature phase shift key suppressed carrier wherein the phase of the received signal components independently represents a consistent stream of binary values of the data source.